Some of thermal printheads for performing printing on a recording medium such as thermal paper or a thermal transfer ink ribbon include an external connection member which is utilized for connection to an external device and connected, by soldering, to a substrate provided with a heat-producing resistor and a drive IC.
FIG. 10 is a sectional view showing a principal portion of such a printhead. The thermal printhead X includes a substrate 91 to which a flexible cable 95 as the external connection member is connected. A glaze layer 92 is provided on the substrate 91. On the glaze layer 92, a wiring 93 constituting a circuit is formed. A plurality of electrodes 94 are provided at appropriate portions of the wiring 93. The flexible cable 95 includes a resin substrate 95a and a plurality of conductor lines 95b formed on the resin substrate. Each of the conductor lines 95b is connected directly to a respective one of the electrodes via solder 98.
An end of the flexible cable 95 is covered, along with part of the substrate 91, by a resin layer 97 for preventing detachment from the substrate 91. With this structure, even when stress from the outside or thermal stress in driving is exerted, the separation of the flexible cable 95 from the electrode 94 and the resulting unstable connection between these members can be avoided.
However, since the solder 98 contracts when it cools and solidifies, the contraction force of the solder 98 exerts on the electrode 94 and the glaze layer 92 to produce a stress. Such a stress may cause the detachment of the electrode 94 or the breakage of the glaze layer 92, which may lead to disconnection between each of the conductor lines 95b and the drive IC (not shown) connected to the line. Therefore, the reliability of connection of the flexible cable 95 may be deteriorated.
Patent Document 1: JP-A-H07-30218